Filtration of a fluid, such as a gas in a compressed gas system is generally required so that the fluid is sufficiently free of contaminants (liquid, solid or gaseous) for a subsequent application or to minimise adverse effects of contaminants on components of the system. For example, removal of compressor oil from a compressed gas stream can be required to minimise chemical contamination and accumulation on valves which might lead to malfunction of the valves. It is known to treat a fluid stream to remove contaminants (liquid, solid or gaseous) by filtering the fluid stream, using a filter element, also known as a coalescing filter. It is also known to treat a fluid stream by imparting a helical flow to fluid entering the filter assembly, using a flow director, so that centrifugal forces causes separation of bulk liquid or solids, such as water, from the stream.
A filter device, such as a filter element or a flow director, used for such filtration or separation can require maintenance and replacement. In particular, filter elements have a limited lifetime and the accumulation of contaminants with prolonged use gives rise to unacceptable pressure drop, which can lead to reentrainment of the contaminants in the fluid stream. It is therefore necessary to replace the filter element from time to time.
Filter assemblies typically comprise a housing comprising head and body parts, and a filter element which can be fitted within the housing. Such filter elements commonly have a wall of a filtration medium and an end cap with an inlet which can be sealed to the head part of the housing to provide a flow path for a fluid stream to be supplied to the space within the filter element. The inlet is provided by a port on an end cap of the element. The port has an O-ring seal on its external surface which is received in a bore within the housing end cap, in which it is compressed to form a seal.
The filter element is assembled in the housing by locating the port on the element end cap in the bore in the housing head part. The O-ring seal is compressed as a result of this, so that a seal is formed between the bore in the housing head part and the element. The head and body parts of the housing can then connected together by means of cooperating screw threads.
The head and body parts of the housing are separated in order to gain access to the filter element when it has to be replaced. The frictional engagement between the O-ring seal and each of the bore in the housing head part and the flow conduit on the element end cap means that the filter element remains attached to the housing head part when the housing body part is removed. This effect in a compressed O-ring seal is sometimes referred to as “stiction”. The element has to be gripped in order to separate it from the housing head part. This can involve gripping the filter media. The media can often be loaded with collected oil and other impurities, which can give rise to cleanliness and contamination or safety issues for the user.